1. Field of the Invention
This invention relates to an ignition timing control system for an internal combustion engine having a function of detecting a knock in the engine to advance or retard an angle for ignition timing. The system will hereinafter be referred to as a knock control system.
2. Description of the Prior Art
In recent years, the knock control system which detects a knock occurring in the engine to advance or retard an angle for ignition timing in accordance with a state of the knock has been discussed from various standpoints of view. By using the knock control system, it is possible to constantly control the ignition timing toward neighborhood of a limit for the knock so that fuel consumption and output performance of the engine can be utilized at a maximum efficiency independently of irregularity and aging of the engine.
A prior art knock control system, however, faces a fatal problem that engine running is degraded during acceleration, or the acceleration cannot be achieved perfectly. More particularly, the ignition timing is retarded considerably because of consecutive generation of knocks immediately after initiation of acceleration, resulting in a delayed acceleration. This problem is attributable to the fact that the ignition timing required for a transient condition, especially, a rapid acceleration varies abruptly with the result that speeds of advance angle operation by the knock control system cannot follow the abrupt variation. As well known in the art, as amounts of advance angle averaged over a unit time or a unit cycle (hereinafter referred to as advance angle rate) increase, so that knock control system needs to follow critically the transient condition. With an increased advance angle rate, however, the ignition timing under normal running of the engine varies to a great extent and torque varies excessively, resulting in degradation of drive-ability under the normal running. For these reasons, the prior art system adopts an advance angle rate of, for example, 1.degree. CA (crank angle)/second which is a compromise between the normal condition and transient condition, failing to satisfy compatibility with both the normal running and the transient condition (especially with transient performance). In view of the above, in order to present an essential solution to elimination of delay in acceleration, it is necessary to increase the advance angle rate only during acceleration and to maintain a small advance angle during normal running.
A countermeasure therefor is conceivable wherein the acceleration state is judged by detecting engine conditions such as for example engine speed and manifold pressure and the advance angle rate is varied on the basis of the detection results. However, the knock condition in the engine, accordingly, the retard angle condition for ignition timing greatly depends on environmental conditions, aging and the like. Therefore, if the advance angle rate is increased indiscreetly on the basis of acceleration information derived from the engine conditions, a rapid advance angle operation will be effected even when the ignition timing is substantially in condition for the normal running, giving rise to frequent generation of excessive knocks and excessive variation in torque. Such inconveniences are due to the effectuation of the advance angle rate in disregard of the knock state or retard angle condition under acceleration and following knock state or advance angle condition.